Integrated circuits (ICs) are often implemented by using a plurality of interconnected semiconductor devices, such as field effect transistors (FETs). More and more semiconductor devices are incorporated on a single IC chip with the development of the device fabrication technology, and thus the size of each device on the IC chip and the spacing between the devices (i.e., feature size) continue to decrease. The individual devices of the ICs, such as FETs and other passive and active circuit elements, are usually interconnected by metal or other conductors to implement desired circuit functions. A small contact resistance is associated with each contact between the conductor and the circuit element. As the feature size continues to decrease, the contact resistance associated with an individual circuit element often increases and becomes more important with respect to the total circuit resistance. In many circumstances, reducing the contact resistance of the devices may boost the performance of the ICs.
Gas cluster ion beam (GCIB) is often used in processing semiconductor devices. For example, in GCIB, gas phase atomic clusters containing thousands of atoms/molecules (e.g., O2, SiH4) are often created by supersonic expansion, and then weakly ionized. The ions accelerated to impact a substrate surface. A large amount of ions interact nearly simultaneously with the substrate atoms/molecules. Consequently, a large amount of energy is received in a relatively small volume near the substrate surface, which leads to extreme chemical and physical reactions at the substrate surface. However, individual ions in GCIB have low energy (e.g., a few eV) and thus, few ions can penetrate deeply into the substrate. For example, a pico-second temperature/pressure spike and thus infusion effects (e.g., melting) may occur within a distance of 2-20 nm from the substrate surface upon the impact of the ions in GCIB, while the bulk of the substrate remains at room temperature.